Internal combustion engine idle air control systems are known to precisely meter inlet air to cylinders of the engine in response to a difference between actual engine speed and a relatively low target engine speed. Change in engine torque load, for example resulting from change in accessory load, can perturb engine speed away from the target speed. A desirable engine idle air control system can reject typical changes in engine torque load to provide a stable engine speed that is pleasing to a vehicle operator.
Certain engine torque load changes can occur very rapidly. For example, substantially a step change in torque load can occur during certain transient maneuvers. To reject such rapid torque load changes, an engine idle air control system must be very responsive. Effort has been made in conventional systems to assess system response limits and then to push system response to the limits in response to a detected rapid torque load change to provide precise transient control of idle speed throughout the load change or, more specifically, to minimize the difference between the target and actual engine speeds throughout the transient maneuver.
It has been determined that the response limit of an engine idle air control system changes with change in barometric pressure. It has further been determined that the response requirements of the idle air control system change with change in barometric pressure. More specifically, in engine idle air control systems including an actuator coupled to a valve for metering air to the engine, the speed at which the actuator can be accurately driven is constrained by the load acting against the valve. The load acting against the valve increases with increasing barometric pressure. Conventional engine idle air control systems prescribe a fixed actuator speed over all barometric pressures. Accordingly, to provide for accurate actuator control, a worst case actuator speed is prescribed, which is a speed determined under high barometric pressure conditions. In such systems, untapped actuator performance capability remains under barometric pressure conditions below such high barometric pressure conditions.
To reject an engine load change, a corresponding change in engine torque is administered by changing an amount of fuel and air admitted to engine cylinders. The engine idle air control system provides for a desired time rate of change in intake air. The fuel control system reacts to the time rate of change in intake air to provide a corresponding time rate of change in injected fuel. As barometric pressure changes, the change in air density affects the amount of intake air passed to the engine for a given state of the idle air control system. As barometric pressure increases, for example, more air is admitted to the engine for a given state, and less change in position of a valve of the engine idle air control system is needed to provide for a desired time rate of change in intake air. Conventional systems prescribe, in response to a change in engine load, a change in position of the valve of the idle air control system at a single rate, which leads to, under varying barometric pressure conditions, significant variation in the time rate of change in air admitted to the engine. Substantially sub-optimized idle air control performance results, for example when responding to an engine load change under barometric pressure that is significantly different than a calibration pressure.
It would therefore be desirable to account for the effect of change in barometric pressure in engine idle air control, to maximize, when necessary, the responsiveness of the control, and to improve control accuracy.